JP3035084B2 - Ultra high strength galvanized steel sheet without hydrogen embrittlement - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high strength galvanized steel sheet suitable for applications requiring light weight and high corrosion resistance, such as bumpers for automobiles, reinforcing members for doors, and scaffolds for construction. is there.

[0002]

2. Description of the Related Art CAFE (CorporateAverage
FuelEconomy)
Has advanced to 1180 N /
mm^TwoThe above ultra-high strength thin steel sheets have been adopted
Was. These steel sheets are non-plated and have a problem with corrosion resistance.
Recently, zinc plating is considered to improve its rust prevention ability
It has become.

However, when galvanizing such an ultra-high-strength steel sheet, for example, in electrogalvanizing, hydrogen generated by a cathodic reaction in pickling and a plating bath before plating enters the steel sheet, It is known that in plating, hydrogen in the heating atmosphere of the line enters the steel sheet, and in any case, hydrogen embrittlement occurs. It is known that such hydrogen embrittlement easily occurs at a tensile strength of 1180 N / mm ² or more, similarly to delayed fracture.

[0004] It has been reported that hydrogen embrittlement of a steel sheet is caused by this diffusible hydrogen, and this hydrogen is reduced by heating at 200 ° C for several hours (for example, “Metal Surface Technology” Vol. 39, No. .7, 1988, p.52). However, it is difficult to completely eliminate diffusible hydrogen in the steel even by heat-treating the steel sheet in this way, and hydrogen embrittlement cannot be prevented. Further, when the steel sheet is heat-treated after galvanizing, the galvanized layer is deteriorated, and the intended corrosion resistance cannot be obtained.

[0005]

As described above, conventionally,
There was no galvanized steel sheet having a good corrosion resistance and a tensile strength of 1180 N / mm ² or more that did not cause hydrogen embrittlement.

In view of the above-mentioned situation, the present invention has a tensile strength of 1180 N / mm which does not cause hydrogen embrittlement with a predetermined zinc or zinc-based plating without reheating treatment for releasing hydrogen. It is intended to provide ^two or more steel sheets.

[0007]

Means for Solving the Problems As means for solving the above-mentioned problems, the present invention relates to a steel plate in which pure zinc or zinc-based plating is applied to the front and back surfaces of an ultra-high strength steel plate having a tensile strength of 1180 N / mm ² or more. , The number of cracks connected from the interface between the steel sheet and the plating layer to the plating surface is 1 / mm ² or more and 10 ⁷ / mm ²
It is characterized in that it exists in the following range.

Hereinafter, the present invention will be described in more detail.

[0009]

[Action]

[0010] The present inventors have found, after extensive research on the cause and countermeasures hydrogen embrittlement in ultra high-strength steel sheet, pure zinc on the front and back surfaces of the tensile strength of 1180N / mm ² or more ultra-high strength steel plate or In a steel plate with zinc plating,
It has been found that hydrogen embrittlement does not occur when cracks connected from the interface between the steel sheet and the plating layer to the plating surface are present in the range of 1 / mm ² or more and 10 / mm ² or less. Although the reason for this is not clear, it is considered that hydrogen contained in the steel sheet is released outside the steel sheet through cracks during the plating process.

When the number of cracks is less than 1 / mm ² , the release of hydrogen from the steel sheet is delayed, and hydrogen cracks occur. On the other hand, if it is more than 10 ⁷ / mm ² , peeling of the plating layer is likely to occur, and the corrosion resistance and the like are undesirably deteriorated. The width of the crack is not particularly limited, and may be any size as long as hydrogen can diffuse. Also, the method for forming the crack is not particularly limited, either.
It may be introduced by adjusting the heat treatment conditions of the plating layer or by a mechanical method such as skin pass or tension after galvanizing.

The type and method of galvanizing are not particularly limited. Pure zinc plating includes electrogalvanizing and hot-dip galvanizing. Examples of the zinc-based plating include galvanizing with an electric alloy such as Zn-Ni and Zn-Mn, and galvannealing. Regardless of whether the galvanized steel sheet is a single layer or a composite layer composed of two or more types of plating, the surface of the plating layer is further subjected to various surface treatments, ie, chromate treatment or phosphoric acid. The effect of the present invention does not change even if salt treatment is performed.

The type of the steel plate to be plated is not particularly limited either, and may be a cold-rolled high-strength steel plate or a hot-rolled high-strength steel plate. It may be a coil material or a sheet material.

Next, an embodiment of the present invention will be described.

[0015]

[Example] Cold rolled steel sheet whose transformation structure has been strengthened by a continuous annealing furnace
(1.6 mm thick) and hot-rolled steel sheets (2.4 mm thick) utilizing transformation structure strengthening and precipitation strengthening, various galvanized steel sheets shown in Table 1 with a length of 70 mm and a width of 15 mm The test piece was cut by various methods, the same strain as that of the coil was imparted by four-point bending, and the sample was kept at room temperature for one week in the air.
And the number of cracks in the plating layer were examined. The results are also shown in Table 1.

[0016]

[Table 1]

The method of forming a crack in the plating layer is as follows. For CG-A of steel No. 3, No. 8 to No. 10, the Fe concentration in the galvanized layer is increased to about 11 mas%, and then the V leveler is increased. Steel No. 4 to No. 7 adopted a method using a V leveler and a skin pass, and the number of cracks was adjusted by changing the rolling conditions. Hydrogen embrittlement was evaluated by immersing hydrochloric acid in a sample to which strain was imparted by four-point bending, and determining whether or not there was breakage. The powdering property was evaluated based on the amount of zinc peeled at a 45 ° V-bent portion.

In Table 1, since steel No. 1 has low strength, hydrogen embrittlement does not occur while the shear is cut on both sides. Steel N
Both No. 2 and No. 8 are conventional methods, and hydrogen embrittlement occurs because the number of cracks in the plating layer is not appropriate.
On the other hand, steels No. 3, No. 7, No. 9 and No. 10 are all examples of the present invention, and have an appropriate number of cracks in the plating layer and no hydrogen embrittlement.

[0018]

As described in detail above, according to the present invention,
Hydrogen embrittlement can be prevented even for a steel plate having a tensile strength of 1180 N / mm ² or more and plated with pure zinc or zinc.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-236753 (JP, A) JP-A-2-263971 (JP, A) JP-A-51-41642 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. A steel sheet having ultra-high strength steel sheet having a tensile strength of 1180 N / mm ² or more, in which pure zinc or zinc-based plating is applied to the front and back surfaces, has one crack connected from the interface between the steel sheet and the plating layer to the plating surface. An ultrahigh-strength galvanized steel sheet free from hydrogen embrittlement, characterized by being present in a range of not less than 10 / mm ^{2 and not} more than 10 ⁷ / mm ² .